The Forest of Essex was a royal forest that existed from around 1100 and was disestablished in the 13th century.
Forests were legal institutions introduced by the Normans to denote an area where the King or another magnate had the right to keep and hunt deer and make Forest Law. Initially there was a very weak correlation between the extent of the legal forest and what might be termed the 'physical forest', the often wooded common land areas where the deer lived. In later centuries there was a much stronger correlation, so much so that the word forest is now taken to mean the same as woodland.
The Forest of Essex covered nearly all of the traditional extent of Essex, but that is not to say the majority of Essex was wooded. The naturalist Oliver Rackham carried out an analysis of Domesday returns for Essex and was able to estimate the county was 20% wooded in 1086. The area covered by Forest Law excluded the least wooded areas of the county along the Thames and North Sea coasts so the percentage for the Forest of Essex was a little higher.
The disestablishment of the Forest of Essex led to the creation of four new smaller Forests which concentrated on areas with a greater proportion of woodland cover, namely: Waltham Forest (which included the physical forest areas subsequently known as Epping Forest and Hainault Forest), Hatfield Forest, Writtle Forest and the long lost Kingswood Forest near Colchester.